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Thursday, 14 July 2011

Checking alignment

The observing frequency for the High-Band Array is 120 to 240 MHz. That's a wavelength of 2.50 to 1.25 metres. In order to achieve good beamforming and interferometric performance, the positional tolerance of the receiver elements still needs to be a small fraction of a wavelength. That means that KAIRA still has a tight error budget. And, given that the positional accuracy is only one factor, and that other aspects of the design such as cable delay errors and timing can also affect the performance, additional accuracy is required. Thus, the KAIRA antenna tiles need to be positioned to ±3cm.

Over long distances, this requires a fair amount of surveying work, not just for the initial marling out of the locations, but also in checking the work that has been done. In the photograph to the left, Mikko Tilja and Tero Raita check the alignment of a sighting laser. This is used to ensure a straight edge along the centre block of antenna tiles. Despite a good local accuracy, we discovered a small rotational error which, if left uncorrected, would result in a large error at the far end of the field.

The amount of corrective work required has been minimal, but it is still useful to cross check all the work done so far to ensure the best possible performance from the array.

Additional work is planned for the vertical position of the antennas. Again, although the field looks flat, we need it to be good to a centimetre across the field, which is a pretty tight tolerance to be required.